Polymer blend proton exchange membrane and method for manufacturing the same

ABSTRACT

The present invention relates to a polymer blend proton exchange membrane comprising a soluble polymer and a sulfonated polymer, wherein the soluble polymer is at least one polymer selected from the group consisting of polysulfone, polyethersulfone and polyvinylidene fluoride, the sulfonated polymer is at least one polymer selected from the group consisting of sulfonated poly(ether-ether-ketone), sulfonated poly(ether-ketone-ether-ketone-ketone), sulfonated poly(phthalazinone ether ketone), sulfonated phenolphthalein poly (ether sulfone), sulfonated polyimides, sulfonated polyphosphazene and sulfonated polybenzimidazole, and wherein the degree of sulfonation of the sulfonated polymer is in the range of 96% to 118%. The present invention further relates to a method for manufacturing the polymer blend proton exchange membrane.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of international Patent ApplicationNo. PCT/CN2009/001372, filed Dec. 4, 2009, which is incorporated hereinby reference

TECHNICAL FIELD

The present invention relates to a polymer blend proton exchangemembrane and the manufacture method thereof. In particular, the presentinvention relates to a polymer blend proton exchange membrane comprisinga soluble polymer and a sulfonated polymer with proton exchangefunction. The polymer blend proton exchange membrane according to thepresent invention can be used in redox flow battery, particularly invanadium redox flow battery.

BACKGROUND OF THE INVENTION

Energy crisis and environment pollution are two big problems forachieving sustainable development of global economics. An efficient wayto solve the two problems is to develop more effective renewable energysuch as wind energy, solar energy, tidal energy, etc. To ensure stablesupply of renewable energy such as wind energy, solar energy, etc., anenergy storage technology with high capacity, low cost, high efficiency,and high reliability and without pollution must be developed. Therefore,one of hot spots in the world in energy field is to develop an energystorage system with high capacity.

Among various energy storage systems with high capacity, vanadium redoxbattery (VRB) has been put into demonstrative operation in wind powergeneration, solar power generation, and the peak regulation of powergrid and the like in foreign countries because of its unique advantagessuch as long lifetime, high reliability, low cost for operation andmaintenance, and the like.

Vanadium battery uses solutions of vanadium ions with different valencesas active species, wherein V⁴⁺/V⁵⁺ redox couple is used for positiveelectrode and V²⁺/V³⁺ redox couple is used for negative electrode.During a charge process, V⁴⁺ is changed into V⁵⁺ at positive electrodeand V³⁺ is changed into V²⁺ at negative electrode. During a dischargeprocess, V⁵ is changed into V⁴⁺ at positive electrode and V²⁺ is changedinto V³⁺. A cell of vanadium battery is comprised of a bipolar plate,electrodes and a separator membrane, wherein the separator membrane ofvanadium battery must be able to prevent vanadium ions of differentvalences in the electrolytes for the positive electrode and for thenegative electrode from permeating through the separator membrane butpermitting the transfer of proton of hydrogen through the separatormembrane. Therefore, the separator membrane should have a desirableproton conductivity as well as a high selective permeability forprotons. Furthermore, the separator membrane must have a long-termchemical stability and good mechanical properties so as to meet the longlife-time requirement of vanadium battery.

Currently, the common used membrane in vanadium battery is theperfluorosulfonic acid proton exchange membrane provided by DuPontCompany under the trade name of Nafion. Perfluorosulfonic acid protonexchange membrane has excellent chemical stability and ion conductivityand can meet the requirement of vanadium battery. However,Perfluorosulfonic acid proton exchange membrane has poor permselectivityand vanadium ions can permeate through the membrane during the operationof vanadium battery. Thus, the self-discharge of vanadium battery occursand the capacity of vanadium battery is reduced. Furthermore, the highcost of perfluorosulfonic acid proton exchange membrane is one of thefactors obstructing the large scale commercialization of vanadiumbattery. Therefore, an important step for the commercialization ofvanadium battery is to develop a proton exchange membranes suitable foruse in vanadium battery with low cost, high chemical stability, good ionconductivity, high permselectivity and high mechanical strength.

In the field of fuel battery, in order to reduce the cost of protonexchange membrane, some non-flouorous hydrocarbon polymers areextensively studied to be used as the membrane forming material afterbeing sulfonated. Such kinds of polymers generally have properties suchas high chemical and thermal stability, and low cost, for example,polyethersulfone, poly (ether ketone), polyimide, polyphosphazene,polybenzimidazole, etc. Theses polymers are sulfonated to form protonexchange membranes, and the resulting membranes have a property that theproperties thereof such as proton conductivity of the membrane depend ona degree of sulfonation of the polymer. The degree of sulfonation of thepolymer shall be high enough so that an ideal conductivity is obtained.However, when the degree of sulfonation of the polymer is high, amechanical property and dimensional and chemical stabilities of themembrane will become poor and thus the requirement of usage will not besatisfied. The proton exchange membrane used in the vanadium battery canalso be made from these sulfonated polymers. However, these membraneswill face a similar problem, i.e., how to compromise among degree ofsulfonation, ion conductivity and chemical stability, mechanicalstrength, and vanadium ion permeability.

SUMMARY OF THE INVENTION

The applicant of the present invention has surprisingly found that aproton exchange membrane with excellent comprehensive properties can bemanufactured by blending a polymer with high degree of sulfonation and asoluble polymer.

Therefore, one object of the present invention is to provide a polymerproton exchange membrane comprising a soluble polymer and a sulfonatedpolymer, wherein the soluble polymer is at least one polymer selectedfrom the group consisting of polysulfone (PS), polyethersulfone (PES)and polyvinylidene fluoride (PVDF), the sulfonated polymer is at leastone polymer selected from the group consisting of sulfonatedpoly(ether-ether-ketone) (SPEEK), sulfonatedpoly(ether-ketone-ether-ketone-ketone) (SPEKEKK), sulfonatedpoly(phthalazinone ether ketone) (SPPEK), sulfonated phenolphthaleinpoly (ether sulfone), sulfonated polyimides (SPI), sulfonatedpolyphosphazene and sulfonated polybenzimidazole (PBI), and wherein thedegree of sulfonation of the sulfonated polymer is in the range of 96%to 118%.

In the context of this disclosure, the term “soluble polymer” means thatthe polymer is soluble in organic solvent. The organic solvent includes,but not limits to, one or more of dimethylacetamide, dimethylfomamide,dimethyl sulfoxide, triethyl phosphate, cyclopentanone,N-methyl-2-pyrrolidone, tetramethylurea, and propylene carbonate.Preferably, the organic solvent is selected from one or more ofN,N-dimethylfomamide, N,N-dimethylacetamide, and N-methyl-2-pyrrolidone.

In a preferable embodiment of the present invention, the degree ofsulfonation of the sulfonated polymer is in the range of 98% to 116%,preferably 100% to 114%, more preferably 106% to 110%.

In another preferable embodiment of the present invention, thesulfonated polymer is made from the unsulfonated polymer which has amelt viscosity in the range of 100 to 550 Pa·s, preferably 300 to 450Pa·s, more preferably 350 to 400 Pa·s, at 300° C. to 500° C.

Preferably, the sulfonated polymer is obtained by directly dissolvingthe unsulfonated polymer in concentrated sulfuric acid, Nordhausen acid,or chlorosulfonic acid and sulfonating the unsulfonated polymer.

Preferably, the sulfonated polymer is prepared by directly dissolvingthe unsulfonated polymer in concentrated sulfuric acid and sulfonatingthe unsulfonated polymer. The concentrated sulfuric acid is used in anamount of 2 to 15 ml of concentrated sulfuric acid per gram ofunsulfonated polymer, preferably 5 to 7 ml of concentrated sulfuric acidper gram of unsulfonated polymer.

Preferably, the sulfonated polymer is prepared by two steps: the firststep is to carry out the reaction for 3 to 5 hours at 20 to 40° C.; thesecond step is to carry out the reaction for 1 to 4 hours at 70 to 100°C.

Preferably, the resulting sulfonated polymer is shaped through awater-cooling process, preferably through the following process: theresulting sulfonated polymer slurry is poured into a screen with 1 to 4mm mesh size and the slurry flows into deionized water beneath thescreen and the strip-like sulfonated polymer is obtained after stirring.

Preferably, the resulting strip-like sulfonated polymer is washed toremove remaining sulfuric acid, then is dried at a temperature of 100 to120° C. for at least 1 hour, preferably at least 4 hours, so as toremove the water sufficiently.

In a further preferable embodiment of the present invention, theweight-average molecular weight of the soluble polymer is in the rangeof 35000 to 65000, preferably 45000 to 55000, more preferably 48000 to52000.

In a still further preferable embodiment of the present invention, thecontent of the soluble polymer is 10% to 50%, preferably 13% to 38%,more preferably 18% to 35%, most preferably 22% to 32%, based on thetotal weight of the membrane.

The thickness of the polymer blend proton exchange membrane according tothe present invention has no specific limitation, but can be determinedbased on the operating requirements, preferably is in the range of 30 to200 μm, more preferably 50 to 100 μm.

Another object of the present invention is to provide a method formanufacturing a polymer proton exchange membrane, comprising thefollowing steps:

-   -   a) dissolving a soluble polymer in an organic solvent and        obtaining a uniform solution, wherein the soluble polymer is at        least one polymer selected from the group consisting of        polysulfone (PS), polyethersulfone (PES) and polyvinylidene        fluoride (PVDF);    -   b) dissolving a sulfonated polymer in the solution obtained in        step a) and obtaining a membrane forming solution, wherein the        sulfonated polymer is at least one polymer selected from the        group consisting of sulfonated poly(ether-ether-ketone),        sulfonated poly(ether-ketone-ether-ketone-ketone), sulfonated        poly(phthalazinone ether ketone), sulfonated phenolphthalein        poly (ether sulfone), sulfonated polyimides, sulfonated        polyphosphazene and sulfonated polybenzimidazole, and wherein        the degree of sulfonation of the sulfonated polymer is in the        range of 96% to 118%.    -   c) forming a membrane by tape casting the membrane forming        solution, drying and heat-treating the membrane, then peeling        off the membrane.

Preferably, the method according to the present invention can include afurther step: d) immersing the membrane in sulfuric acid aqueoussolution for a whole day to make the membrane fully protonated. Stillpreferably, the concentration of the sulfuric acid aqueous solution usedin the step d) of the method according to the present invention is inthe range of 0.5 to 1.5 M and the immersion time is in the range of 15to 30 hours.

In a preferable embodiment according to the present invention, theorganic solvent includes, but not limits to, one or more ofdimethylacetamide, dimethylfomamide, dimethyl sulfoxide, triethylphosphate, cyclopentanone, N-methyl-2-pyrrolidone, tetramethylurea, andpropylene carbonate. Preferably, the organic solvent is selected fromone or more of N,N-dimethylfomamide, N,N-dimethylacetamide, andN-methyl-2-pyrrolidone.

In a preferable embodiment according to the present invention, thedegree of sulfonation (DS) of the sulfonated polymer is in the range of98% to 116%, preferably 100% to 114%, more preferably 106% to 110%.

In another preferable embodiment according to the present invention, thesulfonated polymer is made from an unsulfonated polymer which has a meltviscosity in the range of 100 to 550 Pa·s, preferably 300 to 450 Pa·s,more preferably 350 to 400 Pa·s, at 300° C. to 500° C.

Preferably, the sulfonated polymer is prepared by directly dissolvingthe unsulfonated polymer in concentrated sulfuric acid, Nordhausen acid,or chlorosulfonic acid and sulfonating the unsulfonated polymer.

Preferably, the sulfonated polymer is prepared by directly dissolvingthe unsulfonated polymer in concentrated sulfuric acid and sulfonatingthe unsulfonated polymer. The concentrated sulfuric acid is used in anamount of 2 to 15 ml of concentrated sulfuric acid per gram ofunsulfonated polymer, preferably in an amount of 5 to 7 ml ofconcentrated sulfuric acid per gram of unsulfonated polymer.

Preferably, the sulfonated polymer is prepared by two steps: the firststep is to carry out the reaction for 3 to 5 hours at 20 to 40° C.; thesecond step is to carry out the reaction for 1 to 4 hours at 70 to 100°C.

Preferably, the resulting sulfonated polymer is shaped through awater-cooling process, preferably through the following process. Theresulting sulfonated polymer slurry is poured into a screen with 1 to 4mm mesh size and the slurry flows into deionized water beneath thescreen, obtaining a strip-like sulfonated polymer after stirring.

Preferably, the resulting strip-like sulfonated polymer is washed toremove the remaining sulfuric acid, then is dried at a temperature of100 to 120° C. for at least 1 hour, preferably at least 4 hours, so asto remove the water thoroughly.

In a further preferable embodiment according to the present invention,the weight-average molecular weight of the soluble polymer is in therange of 35000 to 65000, preferably 45000 to 55000, more preferably48000 to 52000.

In a still further preferable embodiment according to the presentinvention, the content of the soluble polymer is 10% to 50%, preferably13% to 38%, more preferably 18% to 35%, most preferably 22% to 32%,based on the total weight of the membrane.

The thickness of the polymer blend proton exchange membrane according tothe present invention has no specific limitation, but can be determinedbased on the operating requirements, preferably is in the range of 30 to200 μm, more preferably 50 to 100 μm.

The proton exchange membrane according to the present invention can beprepared by well known membrane forming processes such as tape casting,casting, and the like without any specific requirement.

In addition to the use in redox flow battery, in particular in vanadiumredox flow battery, the proton exchange membrane according to thepresent invention can also be used in proton exchange membrane fuelbattery, in particular direct methanol fuel cell.

Preparation and Selection of Sulfonated Polymer

According to the present invention, a proton exchange membrane withexcellent comprehensive properties is manufactured by blending a polymerwith high degree of sulfonation and a soluble polymer. The sulfonatedpolymer with high degree of sulfonation has excellent protonconductivity, but has poor mechanical properties and poor dimensionalstability. Without bound to any specific theory, cross-linking betweenthe sulfonated polymer according to the present invention and theblended soluble polymer can occur, and such cross-linking will obstructswelling of the sulfonated polymer so that the mechanical properties anddimensional stability are improved and the vanadium ion permeabilitythrough the proton exchange membrane is lowered. Preferably, the degreeof sulfonation (DS) of the sulfonated polymer according to the presentinvention is in the range of 96% to 118%, preferably 98% to 116%, morepreferably 100% to 114%, still more preferably 106% to 110%, so as toensure the electric conductivity of the sulfonated polymer. Herein, thedegree of sulfonation means the number of sulfonic acid groups containedin 100 repeating unit.

The sulfonated polymer used in the proton exchange membrane according tothe present invention can be same kind of sulfonated polymers with avariety of degrees of sulfonation.

The sulfonated polymer with high DS used in the proton exchange membraneaccording to the present invention is made from an unsulfonated polymerwhich has a melt viscosity in the range of 100 to 550 Pa·s, preferably300 to 450 Pa·s, more preferably 350 to 400 Pa·s, at 300° C. to 500° C.

Preferably, the sulfonated polymer is prepared by directly dissolvingthe unsulfonated polymer in concentrated sulfuric acid, Nordhausen acid,or chlorosulfonic acid and sulfonating the unsulfonated polymer.

Preferably, the sulfonated polymer is prepared by directly dissolvingthe unsulfonated polymer in concentrated sulfuric acid and sulfonatingthe unsulfonated polymer. The concentrated sulfuric acid is used in anamount of 2 to 15 ml of concentrated sulfuric acid per gram ofunsulfonated polymer, preferably in an amount of 5 to 7 ml ofconcentrated sulfuric acid per gram of unsulfonated polymer.

Preferably, the sulfonated polymer is prepared through two steps: thefirst step is to carry out the reaction for 3 to 5 hours at 20 to 40°C.; the second step is to carry out the reaction for 1 to 4 hours at 70to 100° C.

Preferably, the resulting sulfonated polymer is shaped through awater-cooling process, preferably through the following process. Theresulting slurry after sulfonation reaction is poured into a screen with1 to 4 mm mesh size and the slurry flows through the screen mesh intodeionized water beneath the screen, obtaining a strip-like sulfonatedpolymer after stirring.

Preferably, the resulting strip-like sulfonated polymer is washed toremove the remaining sulfuric acid, then is dried at a temperature of100 to 120° C. for at least 1 hour, preferably at least 4 hours, so asto remove the water sufficiently.

The degree of sulfonation can be controlled by selecting the amount ofsulfuric acid used per unit mass of polymer, sulfonation temperature,and/or sulfonation time during sulfonation process. The degree ofsulfonation can be determined by acid-base titration method.

The sulfonated polymer used in the present invention can be prepared byutilizing any other technology well known in the art.

Preparation of Blend Proton Exchange Membrane

According to the method of the present invention, a polymer blend protonexchange membrane can be prepared by the following steps.

A soluble polymer is dissolved in a certain amount of solvent withheating and stirring to obtain a uniform solution. A sulfonated polymeris dissolved in said solution with heating and stirring to obtain auniform membrane forming solution. The membrane forming solution ispoured onto a glass plate to conduct slip casting. The membrane obtainedafter slip casting is put into a drying oven and dried at 50 to 90° C.for 8 to 16 hours, then heat-treated at 80 to 120° C. for 2 to 6 hours.After cooling, the dried membrane together with the glass plate isimmersed into deionized water and the membrane is removed from the glassplate. Then, the membrane is immersed in 0.5 to 1.5 M H₂SO₄ aqueoussolution for 20 to 30 hours, then taken out from the H₂SO₄ aqueoussolution, and washed with deionized water several times to removeremaining H₂SO₄ in the membrane before immersed into deionized water.The thickness of the proton exchange membrane is determined bycontrolling the thickness of tape casted membrane forming solution.

The polymer blend proton exchange membrane according to the presentinvention can be prepared by any other procedure similar to the processof the present invention without departing from the scope of the presentinvention.

The polymer blend proton exchange membrane according to the presentinvention is suitable for use in redox flow battery, in particularvanadium redox flow battery. The polymer blend proton exchange membraneaccording to the present invention has excellent proton conductivity andlow vanadium ions permeability, as well as good mechanical properties,dimensional stability and chemical stability, and low cost.

The polymer blend proton exchange membrane according to the presentinvention has at least one of the following advantages.

-   -   1. The membrane is low cost and easy to obtain. Besides, the raw        materials used in the present invention are commercial products,        and the sulfonation process is easy to operate.    -   2. The membrane has excellent chemical and thermal stability,        and high mechanical strength. Most of the polymer raw materials        used in the present invention are engineering thermoplastics        which have good chemical and thermal stability. The membrane has        good ion conductivity because the sulfonated polymer with high        degree of sulfonation is used.    -   3. The blended polymer selected in the present invention has the        advantages such as low cost, good chemical stability, good        membrane forming property.    -   4. The mechanical property and the dimensional stability of the        membrane have been greatly improved since the swell of the blend        membrane is effectively limited by the cross-linking between        polymers.    -   5. Because of the ion passages of sulfonated polymer itself        which is smaller than those of perfluorosulfonic acid membrane,        as well as the cross-linking effect of the blended polymer, the        capability of the blend membrane to obstruct vanadium ions from        permeating the blend membrane is better than that of        perfluorosulfonic acid membrane.    -   6. Compared with perfluorosulfonic acid membrane, the blend        membrane prepared by the method according to the present        invention has a much lower cost, which will definitely promote        the commercialization of vanadium redox flow battery.

DETAILED DESCRIPTION OF THE PREFERABLE EMBODIMENTS

The preparation examples of sulfonated polymer

1. The Preparation of sulfonated poly(ether-ketone)

10 g of poly(ether-ketone) (Victrex PLC, 22G, melt viscosity of 110 Pa·sat 400° C.) is added into a three-neck flask containing 120 ml ofconcentrated sulfuric acid (98%) at room temperature under stirringelectrically. The three-neck flask is then put into a thermostaticwaterbath at a set temperature of 30° C. and allowed to react for 3.5hours. Then the temperature of the thermostatic waterbath is increasedto 75° C. and maintained for 2 hours. After the reaction finishes, theresulting slurry from the reaction in the three-neck flask is pouredinto a screen with 2 mm mesh size made of polypropylene. The slurryflows through the screen mesh in strip shapes and comes into cooldeionized water. Strip-like polymer materials are formed after theslurry contacts with cool deionized water. Then the strip-like polymeris taken out and washed with deionized water several times to remove thefree acid in the polymer until the pH of the water after washing isabout 7. The washed strip-like polymer is placed into a drying oven anddried at 120° C. for 4 hours until the strip-like polymer turnsred-brown. The dried sulfonated poly(ether-ketone) 1 is crushed forlater use. The degree of sulfonation of the sulfonatedpoly(ether-ketone) 1 is measured as 105% by titration method.

10 g of poly(ether-ketone) (Victrex PLC, 22G, melt viscosity of 110 Pa·sat 400° C.) is added into a three-neck flask containing 90 ml ofconcentrated sulfuric acid (98%) at room temperature under stirringelectrically. The three-neck flask is then put into a thermostaticwaterbath at a set temperature of 30° C. and allowed to react for 3.5hours. Then the temperature of the thermostatic waterbath is increasedto 65° C. and maintained for 2.5 hours. After the reaction finishes, theresulting slurry from the reaction in the three-neck flask is pouredinto a screen with 2 mm mesh size made of polypropylene. The slurryflows through the screen mesh in strip shapes and comes into cooldeionized water. Strip-like polymer materials are formed after theslurry contacts with cool deionized water. Then the strip-like polymeris taken out and washed with deionized water several times to remove thefree acid in the polymer until the pH of the water after washing isabout 7. The washed strip-like polymer is placed into a drying oven anddried at 120° C. for 4 hours until the strip-like polymer turnsred-brown. The dried sulfonated poly(ether-ketone) 2 is crushed forlater use. The degree of sulfonation of the sulfonatedpoly(ether-ketone) 2 is measured as 85% by titration method.

2. Preparation of sulfonated poly(ether-ether-ketone)

10 g of poly(ether-ether-ketone) (Victrex PLC, 381G, melt viscosity of381 Pa·s at 400° C.) is added into a three-neck flask containing 100 mlof concentrated sulfuric acid (98%) at room temperature under stirringelectrically. The three-neck flask is then put into a thermostaticwaterbath at a set temperature of 35° C. and allowed to react for 3hours. Then, the temperature of the thermostatic waterbath is increasedto 75° C. and maintained for 3.5 hours. After the reaction finishes, theresulting slurry from the reaction in the three-neck flask is pouredinto a screen with 2 mm mesh size made of polypropylene. The slurryflows through the screen mesh in strip shapes and comes into cooldeionized water. Strip-like polymer materials are formed after theslurry contacts with cool deionized water. Then, the strip-like polymeris taken out and washed with deionized water several times to remove thefree acid in the polymer until the pH of the water after washing isabout 7. The washed strip-like polymer is placed into a drying oven anddried at 120° C. for 4 hours until the strip-like polymer turnsred-brown. The dried sulfonated poly(ether-ether-ketone) 1 is crushedfor later use. The degree of sulfonation of the sulfonatedpoly(ether-ether-ketone) 1 is measured as 98% by titration method.

10 g of poly(ether-ether-ketone) (Victrex PLC, 381G, melt viscosity of381 Pa·s at 400° C.) is added into a three-neck flask containing 80 mlof concentrated sulfuric acid (98%) at room temperature under stirringelectrically. The three-neck flask is then put into a thermostaticwaterbath at a set temperature of 35° C. and allowed to react for 3hours. Then, the temperature of the thermostatic waterbath is increasedto 65° C. and maintained for 3 hours. After the reaction finished, theresulting slurry from the reaction in the three-neck flask is pouredinto a screen with 2 mm mesh size made of polypropylene. The slurryflows through the screen mesh in strip shapes and comes into cooldeionized water. Strip-like polymer materials are formed after theslurry contacts with cool deionized water. Then, the strip-like polymeris taken out and washed with deionized water several times to remove thefree acid in the polymer until the pH of the water after washing isabout 7. The washed strip-like polymer is placed into a drying oven anddried at 120° C. for 4 hours until the strip-like polymer turnsred-brown. The dried sulfonated poly(ether-ether-ketone) 2 is crushedfor later use. The degree of sulfonation of the sulfonatedpoly(ether-ketone) 2 is measured as 68% by titration method.

The polymer blend proton exchange membrane in Examples 1 to 10 andcomparative Examples 1 to 4 are manufactured by using the sulfonatedpolymer with various degrees of sulfonation prepared according to theabove described preparation examples of sulfonated polymer respectively.

Example 1

0.10 g of PVDF powder is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically at room temperature for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.90 g of sulfonated poly(ether-ketone) 1(degree of sulfonation: 105%) prepared according to the above describedpreparation example 1 of sulfonated polymer is weighed and added intothe solution. The vial is placed into a drying oven at 60° C. todissolve the sulfonated poly(ether-ketone) thoroughly before the vial istaken out from the drying oven. A uniform membrane forming solution of12 wt % is obtained after stirring repeatedly. The membrane formingsolution is poured onto a glass plate to conduct tape casting, thendried at 60° C. for 12 hours and heat-treated at 100° C. for 4 hours,then naturally cooled to room temperature. Then, the glass platetogether with the membrane thereon is placed into deionized water. Themembrane is peeled off and immersed in 1 M of sulfuric acid for a wholeday. Then, the membrane is washed with deionized water repeatedly andimmersed in deionized water for later use. The dry thickness of theresulting membrane is 85 μm, the content of PVDF is 10 wt %.

Example 2

0.15 g of PS is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically at room temperature for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.85 g of sulfonated poly(ether-ketone) 1(degree of sulfonation: 105%) prepared according to the above describedpreparation example 1 of sulfonated polymer is weighed and added intothe solution. The vial is placed into a drying oven at 60° C. todissolve the sulfonated poly(ether-ketone) thoroughly before the vial istaken out from the drying oven. Then, a uniform membrane formingsolution of 12 wt % is obtained after stirring repeatedly and dispersingwith ultrasound. The membrane forming solution is poured onto a glassplate to conduct tape casting. Then the membrane formed on the glassplate is dried at 60° C. for 12 hours and heat-treated at 100° C. for 4hours, then naturally cooled to room temperature. Then, the glass platetogether with the membrane thereon is placed into deionized water. Themembrane is peeled off and immersed in 1 M of sulfuric acid for a wholeday. Then, the membrane is washed with deionized water repeatedly andimmersed in deionized water for later use. The dry thickness of theresulting membrane is 82 μm, the content of PS is 15 wt %.

Example 3

0.20 g of PES powder is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically for 30 min to form auniform solution. The solution is filtered to remove any possible smallparticles. 0.80 g of sulfonated poly(ether-ketone) 1 (degree ofsulfonation: 105%) prepared according to the above described preparationexample 1 of sulfonated polymer is weighed and added into the solution.The vial is placed into a drying oven at 60° C. to dissolve thesulfonated poly(ether-ketone) thoroughly before the vial is taken outfrom the drying oven. Then, a uniform membrane forming solution of 12 wt% is obtained after stirring repeatedly and dispersing with ultrasound.The membrane forming solution is poured onto a glass plate to conducttape casting. Then the membrane formed on the glass plate is dried at60° C. for 12 hours and heat-treated at 100° C. for 4 hours, thennaturally cooled to room temperature. Then, the glass plate togetherwith the membrane thereon is placed into deionized water. The membraneis peeled off and immersed in 1 M of sulfuric acid for a whole day.Then, the membrane is washed with deionized water repeatedly and thenimmersed in deionized water for later use. The dry thickness of theresulting membrane is 81 μm, the content of PES is 20 wt %.

Example 4

Powders of 0.10 g of PVDF and 0.15 g of PES are put into a vialcontaining 7.8 ml of N,N-dimethylformamide and stirred magnetically for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.75 g of sulfonated poly(ether-ketone) 1(degree of sulfonation: 105%) prepared according to the above describedpreparation example 1 of sulfonated polymer is weighed and added intothe solution. The vial is placed into a drying oven at 60° C. for 2hours to dissolve the sulfonated poly(ether-ketone) thoroughly beforethe vial is taken out from the drying oven. Then, a uniform membraneforming solution of 12 wt % is obtained after stirring repeatedly anddispersing with ultrasound. The membrane forming solution is poured ontoa glass plate to conduct tape casting. Then the membrane formed on theglass plate is dried at 60° C. for 12 hours and heat-treated at 100° C.for 4 hours, then naturally cooled to room temperature. Then, the glassplate together with the membrane thereon is placed into deionized water.The membrane is peeled off and then immersed in 1 M of sulfuric acid fora whole day. Then, the membrane is washed with deionized waterrepeatedly and then immersed in deionized water for later use. The drythickness of the resulting membrane is 80 μm. The content of PVDF is 10wt % and the content of PES is 15 wt %.

Example 5

Powders of 0.05 g of PVDF, 0.15 g of PES and 0.10 g of PS are put into avial containing 7.8 ml of N,N-dimethylformamide and stirred magneticallyfor 30 min to form a uniform solution. The solution is filtered toremove any possible small particles. 0.70 g of sulfonatedpoly(ether-ketone) 1 (degree of sulfonation: 105%) prepared according tothe above described preparation example 1 of sulfonated polymer isweighed and added into the solution. The vial is placed into a dryingoven at 60° C. for 2 hours to dissolve the sulfonated poly(ether-ketone)thoroughly before the vial is taken out from the drying oven. Then, auniform membrane forming solution of 12 wt % is obtained after stirringrepeatedly and dispersing with ultrasound. The membrane forming solutionis poured onto a glass plate to conduct tape casting. Then the membraneformed on the glass plate is dried at 60° C. for 12 hours and maintainedat 100° C. for 4 hours, then naturally cooled to room temperature. Then,the glass plate together with the membrane thereon is placed intodeionized water. The membrane is peeled off and then immersed in 1 M ofsulfuric acid for a whole day. The membrane is washed with deionizedwater repeatedly and then immersed in deionized water for later use. Thedry thickness of the resulting membrane is 80 μm. The content of PVDF is5 wt %, the content of PES is 15 wt % and the content of PS is 10 wt %.

Example 6

0.10 g of PVDF powder is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically at room temperature for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.90 g of sulfonatedpoly(ether-ether-ketone) 1 (degree of sulfonation: 98%) preparedaccording to the above described preparation example 2 of sulfonatedpolymer is weighed and added into the solution. The vial is placed intoa drying oven at 60° C. to dissolve the sulfonatedpoly(ether-ether-ketone) thoroughly before the vial is taken out fromthe drying oven. A uniform membrane forming solution of 12 wt % isobtained after stirring repeatedly. The membrane forming solution ispoured onto a glass plate to conduct tape casting, then dried at 60° C.for 12 hours and heat-treated at 100° C. for 4 hours, then naturallycooled to room temperature. Then, the glass plate together with themembrane thereon is placed into deionized water. The membrane is peeledoff and immersed in 1 M of sulfuric acid for a whole day. Then, themembrane is washed with deionized water repeatedly and immersed indeionized water for later use. The dry thickness of the resultingmembrane is 85 μm, the content of PVDF is 10 wt %.

Example 7

0.15 g of PS is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically at room temperature for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.85 g of sulfonatedpoly(ether-ether-ketone) 1 (degree of sulfonation: 98%) preparedaccording to the above described preparation example 2 of sulfonatedpolymer is weighed and added into the solution. The vial is placed intoa drying oven at 60° C. to dissolve the sulfonatedpoly(ether-ether-ketone) thoroughly before the vial is taken out fromthe drying oven. Then, a uniform membrane forming solution of 12 wt % isobtained after stirring repeatedly and dispersing with ultrasound. Themembrane forming solution is poured onto a glass plate to conduct tapecasting. Then the membrane formed on the glass plate is dried at 60° C.for 12 hours and heat-treated at 100° C. for 4 hours, then naturallycooled to room temperature. Then, the glass plate together with themembrane thereon is placed into deionized water. The membrane is peeledoff and immersed in 1 M of sulfuric acid for a whole day. Then, themembrane is washed with deionized water repeatedly and immersed indeionized water for later use. The dry thickness of the resultingmembrane is 82 μm, the content of PS is 15 wt %.

Example 8

0.20 g of PES powder is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically for 30 min to form auniform solution. The solution is filtered to remove any possible smallparticles. 0.80 g of sulfonated poly(ether-ether-ketone) 1 (degree ofsulfonation: 98%) prepared according to the above described preparationexample 2 of sulfonated polymer is weighed and added into the solution.The vial is placed into a drying oven at 60° C. for 2 hours to dissolvethe sulfonated poly(ether-ether-ketone) thoroughly before the vial istaken out from the drying oven. Then, a uniform membrane formingsolution of 12 wt % is obtained after stirring repeatedly and dispersingwith ultrasound. The membrane forming solution is poured onto a glassplate to conduct tape casting. Then the membrane formed on the glassplate is dried at 60° C. for 12 hours and heat-treated at 100° C. for 4hours, then naturally cooled to room temperature. Then, the glass platetogether with the membrane thereon is placed into deionized water. Themembrane is peeled off and immersed in 1 M of sulfuric acid for a wholeday. Then, the membrane is washed with deionized water repeatedly andthen immersed in deionized water for later use. The dry thickness of theresulting membrane is 83 μm, the content of PES is 20 wt %.

Example 9

Powders of 0.10 g of PVDF and 0.15 g of PES are put into a vialcontaining 7.8 ml of N,N-dimethylformamide and stirred magnetically for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.75 g of sulfonatedpoly(ether-ether-ketone) 1 (degree of sulfonation: 98%) preparedaccording to the above described preparation example 2 of sulfonatedpolymer is weighed and added into the solution. The vial is placed intoa drying oven at 60° C. for 2 hours to dissolve the sulfonatedpoly(ether-ketone) thoroughly before the vial is taken out from thedrying oven. Then, a uniform membrane forming solution of 12 wt % isobtained after stirring repeatedly and dispersing with ultrasound. Themembrane forming solution is poured onto a glass plate to conduct tapecasting. Then the membrane formed on the glass plate is dried at 60° C.for 12 hours and heat-treated at 100° C. for 4 hours, then naturallycooled to room temperature. Then, the glass plate together with themembrane thereon is placed into deionized water. The membrane is peeledoff and then immersed in 1 M of sulfuric acid for a whole day. Then, themembrane is washed with deionized water repeatedly and then immersed indeionized water for later use. The dry thickness of the resultingmembrane is 80 μm. The content of PVDF is 10 wt % and the content of PESis 15 wt %.

Example 10

Powders of 0.05 g of PVDF, 0.15 g of PES and 0.10 g of PS are put into avial containing 7.8 ml of N,N-dimethylformamide and stirred magneticallyfor 30 min to form a uniform solution. The solution is filtered toremove any possible small particles. 0.70 g of sulfonatedpoly(ether-ether-ketone) 1 (degree of sulfonation: 98%) preparedaccording to the above described preparation example 2 of sulfonatedpolymer is weighed and added into the solution. The vial is placed intoa drying oven at 60° C. for 2 hours to dissolve the sulfonatedpoly(ether-ether-ketone) thoroughly before the vial is taken out fromthe drying oven. Then, a uniform membrane forming solution of 12 wt % isobtained after stirring repeatedly and dispersing with ultrasound. Themembrane forming solution is poured onto a glass plate to conduct tapecasting. Then the membrane formed on the glass plate is dried at 60° C.for 12 hours and maintained at 100° C. for 4 hours, then naturallycooled to room temperature. Then, the glass plate together with themembrane thereon is placed into deionized water. The membrane is peeledoff and then immersed in 1 M of sulfuric acid for a whole day. Themembrane is washed with deionized water repeatedly and then immersed indeionized water for later use. The dry thickness of the resultingmembrane is 80 μm. The content of PVDF is 5 wt %, the content of PES is15 wt % and the content of PS is 10 wt %.

Comparative Example 1

0.10 g of PVDF powder is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically at room temperature for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.90 g of sulfonated poly(ether-ketone) 2(degree of sulfonation: 85%) prepared according to the above describedpreparation example 1 of sulfonated polymer is weighed and added intothe solution. The vial is placed into a drying oven at 60° C. todissolve the sulfonated poly(ether-ketone) thoroughly before the vial istaken out from the drying oven. A uniform membrane forming solution of12 wt % is obtained after stirring repeatedly. The membrane formingsolution is poured onto a glass plate to conduct tape casting. Themembrane formed on the glass plate is dried at 60° C. for 12 hours andheat-treated at 100° C. for 4 hours, then naturally cooled to roomtemperature. Then, the glass plate together with the membrane thereon isplaced into deionized water. The membrane is peeled off and immersed in1 M of sulfuric acid for a whole day. Then, the membrane is washed withdeionized water repeatedly and immersed in deionized water for lateruse. The dry thickness of the resulting membrane is 85 μm, the contentof PVDF is 10 wt %.

Comparative Example 2

Powders of 0.05 g of PVDF, 0.15 g of PES and 0.10 g of PS are put into avial containing 7.8 ml of N,N-dimethylformamide and stirred magneticallyfor 30 min to form a uniform solution. The solution is filtered toremove any possible small particles. 0.80 g of sulfonatedpoly(ether-ketone) 2 (degree of sulfonation: 85%) prepared according tothe above described preparation example 1 of sulfonated polymer isweighed and added into the solution. The vial is placed into a dryingoven at 60° C. to dissolve the sulfonated poly(ether-ketone) thoroughlybefore the vial is taken out from the drying oven. Then, a uniformmembrane forming solution of 12 wt % is obtained after stirringrepeatedly and dispersing with ultrasound. The membrane forming solutionis poured onto a glass plate to conduct tape casting. Then the membraneformed on the glass plate is dried at 60° C. for 12 hours and maintainedat 100° C. for 4 hours, then naturally cooled to room temperature. Then,the glass plate together with the membrane thereon is placed intodeionized water. The membrane is peeled off and then immersed in 1 M ofsulfuric acid for a whole day. The membrane is washed with deionizedwater repeatedly and then immersed in deionized water for later use. Thedry thickness of the resulting membrane is 81 μm. The content of PVDF is5 wt %, the content of PES is 15 wt % and the content of PS is 10 wt %.

Comparative Example 3

0.10 g of PVDF powder is put into a vial containing 7.8 ml ofN,N-dimethylformamide and stirred magnetically at room temperature for30 min to form a uniform solution. The solution is filtered to removeany possible small particles. 0.90 g of sulfonatedpoly(ether-ether-ketone) 2 (degree of sulfonation: 68%) preparedaccording to the above described preparation example 2 of sulfonatedpolymer is weighed and added into the solution. The vial is placed intoa drying oven at 60° C. to dissolve the sulfonatedpoly(ether-ether-ketone) thoroughly before the vial is taken out fromthe drying oven. A uniform membrane forming solution of 12 wt % isobtained after stirring repeatedly. The membrane forming solution ispoured onto a glass plate to conduct tape casting. The membrane formedon the glass plate is dried at 60° C. for 12 hours and heat-treated at100° C. for 4 hours, then naturally cooled to room temperature. Then,the glass plate together with the membrane thereon is placed intodeionized water. The membrane is peeled off and immersed in 1 M ofsulfuric acid for a whole day. Then, the membrane is washed withdeionized water repeatedly and immersed in deionized water for lateruse. The dry thickness of the resulting membrane is 85 μm, the contentof PVDF is 10 wt %.

Comparative Example 4

Powders of 0.05 g of PVDF, 0.15 g of PES and 0.10 g of PS are put into avial containing 7.8 ml of N,N-dimethylformamide and stirred magneticallyfor 30 min to form a uniform solution. The solution is filtered toremove any possible small particles. 0.80 g of sulfonatedpoly(ether-ether-ketone) 2 (degree of sulfonation: 85%) preparedaccording to the above described preparation example 2 of sulfonatedpolymer is weighed and added into the solution. The vial is placed intoa drying oven at 60° C. to dissolve the sulfonatedpoly(ether-ether-ketone) thoroughly before the vial is taken out fromthe drying oven. Then, a uniform membrane forming solution of 12 wt % isobtained after stirring repeatedly and dispersing with ultrasound. Themembrane forming solution is poured onto a glass plate to conduct tapecasting. Then the membrane formed on the glass plate is dried at 60° C.for 12 hours and maintained at 100° C. for 4 hours, then naturallycooled to room temperature. Then, the glass plate together with themembrane thereon is placed into deionized water. The membrane is peeledoff and then immersed in 1 M of sulfuric acid for a whole day. Themembrane is washed with deionized water repeatedly and then immersed indeionized water for later use. The dry thickness of the resultingmembrane is 81 μm. The content of PVDF is 5 wt %, the content of PES is15 wt % and the content of PS is 10 wt %.

The following property tests have been made for Nafion 115 (availablecommercially from DuPont Company, USA) and the polymer blend protonexchange membranes prepared according to the Examples 1 to 10 andComparative Examples 1 to 4.

1. Vanadium Ion Permeability Test of Proton Exchange Membrane

The vanadium ion permeability of proton exchange membrane is conductedwith a permeation cell. A proton exchange membrane is sandwiched betweentwo half cells of the permeation cell, wherein one half cell contains aelectrolyte solution of vanadium battery and the other half cellcontains a sulfuric acid aqueous solution with the same concentration asthat of the electrolyte solution. On testing, the two half cells arestirred simultaneously by electric stirrer. After a certain time, thevanadium ions in the half cell containing the electrolyte solution willenter into the half cell containing the sulfuric acid solution bypermeating the membrane, resulting in the change of the light absorbencyof the sulfuric acid aqueous solution. The relative content of vanadiumions in the sulfuric acid aqueous solution side can be determined bymeasuring the light absorbency of the sulfuric acid aqueous solutionwith ultraviolet-visible spectrometer, thus determining the vanadiumions permeability of various membranes. In the specification, vanadiumions permeability is indicated by the light absorbency of the sulfuricacid aqueous solution after 100 hours.

2. Swell Property Test of Proton Exchange Membrane

Area change rate (ΔS) is used to indicate the swell property of protonexchange membrane. At room temperature, the surface area of wet membrane(S_(w)) is measured after a rectangular membrane sample is immersed inwater for 12 hours. The surface area of dry membrane (S_(d)) is measuredafter the above wet membrane is dried at 80° C. for 12 hours. The areachange rate ΔS is calculated based on the following equation:ΔS=(S _(w) −S _(d))/S _(d)×100%

3. Mechanical Property Test of Proton Exchange Membrane

The mechanical properties of proton exchange membrane are testedaccording to GB 1039-79 and GB 1040-79.

4. Surface Resistance Test of Proton Exchange Membrane

The surface resistance of proton exchange membrane is tested with abattery internal resistance tester using alternating current method. Ontesting, the membrane is sandwiched between the two half cells of apermeation cell. Two graphite electrode plates are respectively fixed onthe two surfaces opposite to the surfaces on which the membrane issandwiched. A V^(3.5+) electrolyte solution (1.7 M V^(3.5+), 2.6 MH₂SO₄) is added into the two half cells up to a predetermined height.After the electrolyte solution become stable, the internal resistance R₂of the permeation cell, i.e. the internal resistance between the twographite electrode plates is measured with the internal resistancetester. The internal resistance R₁ of the permeation cell when themembrane is not sandwiched between the two half cells of the permeationcell is measured under the same conditions. The effective test area ofthe membrane or the opening area of the permeation cell is S. Thesurface resistance of the membrane R (Ω·cm²) is calculated according tothe equation R=(R₁−R₂)×S.

The test results of proton exchange membrane are listed in Table 1.

TABLE 1 Breaking Ultimate Swelling Light Absorbency of Surface Type ofThickness Strength Enlongation Ratio ΔS sulfuric acid solutionResistance@25□ Membrane (μm) (MPa) (%) (%) side after 100 hours (Ω ·cm²) Nafion 115 125 31 446.04 25 0.202 0.42 Example 1 85 45 282.65 220.086 0.38 Example 2 82 43 202.24 17 0.073 0.45 Example 3 81 47 126.0912 0.059 0.48 Example 4 80 38 119.50 8 0.050 0.52 Example 5 80 40 108.216 0.047 0.60 Example 6 85 39 232.30 23 0.077 0.39 Example 7 82 37 208.6918 0.065 0.41 Example 8 83 38 126.48 14 0.056 0.47 Example 9 80 40118.06 8 0.050 0.56 Example 10 80 34 107.88 5 0.043 0.62 Comparative 8542 261.25 22 0.081 0.43 Example 1 Comparative 81 31 97.18 8 0.045 0.95Example 2 Comparative 85 40 201.40 25 0.072 0.48 Example 3 Comparative81 30 67.66 10 0.042 0.98 Example 4

In Table 1, it is indicated that the polymer blend proton exchangemembrane according to the present invention (Examples 1 to 10) exhibitshigher mechanical strength, higher dimensional stability and lowervanadium ion permeability, compared with Nafion 115.

Based on the Examples 1 to 10 of the present invention, the dimensionalstability is improved and vanadium ion permeability is lowered as theamount of the soluble polymer blended is increased.

Moreover, in contrast with the comparative examples 1 to 4 in which thedegree of sulfonation of the sulfonated polymer is beyond the scope ofthe present invention, higher ion conductivity can be obtained byblending more soluble polymer in the polymer blend proton exchangemembrane prepared in Examples 1 to 10 according to the presentinvention. For example, the polymer blend proton exchange membranesprepared in Examples 5 and 10 still have high ion conductivity althoughthe amount of the blended soluble polymer reaches 30%. In contrast, theion conductivity of the membrane in comparative examples 2 and 4decreases dramatically, i.e. the surface resistance increasesdramatically.

Thus, according to the present invention, a polymer blend protonexchange membrane with excellent combination of properties can beobtained by blending a specific soluble polymer in a polymer that isobtained by sulfonating a polymer not containing fluorine. Inparticular, the polymer blend proton exchange membrane according to thepresent invention has an excellent compromise among mechanical property,dimensional stability, vanadium ion permeability and ion conductivity.

Although the present invention has been described in connection with thespecific examples for illustrative purposes, those skilled in the artwill appreciate that various modifications, additions and substitutionsare possible after reading the specification. The present invention isintended to cover all of these modifications, additions andsubstitutions within the scope of the accompanying claims.

The invention claimed is:
 1. A method for manufacturing a polymer blendproton exchange membrane comprising the following steps: a) dissolving asoluble polymer in an organic solvent and obtaining a uniform solution,wherein the soluble polymer is at least one polymer selected from thegroup consisting of polysulfone, polyethersulfone and polyvinylidenefluoride; b) dissolving a sulfonated polymer in the solution obtained instep a) and obtaining a polymer blend membrane forming solution, whereinthe sulfonated polymer is at least one polymer selected from the groupconsisting of sulfonated poly(ether-ether-ketone), sulfonatedpoly(ether-ketone-ether-ketone-ketone), sulfonated poly(phthalazinoneether ketone), sulfonated phenolphthalein poly (ether sulfone),sulfonated polyimide, sulfonated polyphosphazene and sulfonatedpolybenzimidazole, wherein the sulfonated polymer is prepared bydirectly dissolving an unsulfonated polymer in concentrated sulfuricacid, Nordhausen acid, or chlorosulfonic acid and sulfonating theunsulfonated polymer, wherein the sulfonated polymer is prepared by twosteps: the first step is to carry out reaction for 3 to 5 hours at 20 to40° C.; the second step is to carry out reaction for 1 to 4 hours at 70to 100° C., and wherein the degree of sulfonation of the sulfonatedpolymer is in the range of 96% to 118%; and c) forming a polymer blendproton exchange membrane by tape casting the polymer blend membraneforming solution, then removing the polymer blend proton exchangemembrane after being dried and heat-treated.
 2. The method according toclaim 1, wherein the method further includes a step: d) immersing thepolymer blend proton exchange membrane in sulfuric acid aqueous solutionfor a whole day and taking out the membrane for later use.
 3. The methodaccording to claim 1, wherein the degree of sulfonation of thesulfonated polymer is 98 to 116%, preferably 100 to 114%, morepreferably 106 to 110%.
 4. The method according to claim 1, wherein thesulfonated polymer is made from an unsulfonated polymer with meltviscosity of 100 to 550 centipoise, preferably 300 to 450 centipoise,more preferably 350 to 400 centipoise, at 300 to 500° C.
 5. The methodaccording to claim 1, wherein the resulting sulfonated polymer is shapedthrough a water-cooling process, preferably through a process in whichthe resulting sulfonated polymer slurry is poured into a screen with 1to 4 mm mesh size and flows into deionized water beneath the screen anda strip-like sulfonated polymer is obtained after stirring.
 6. Themethod according to claim 5, wherein the resulting strip-shapedsulfonated polymer is washed to remove remaining sulfuric acid, then isdried at a temperature of 100 to 120° C. for at least 1 hour, preferablyat least 4 hours.
 7. The method according to claim 1, wherein theweight-average molecular weight of the soluble polymer is in the rangeof 35000 to 65000, preferably 45000 to 55000, more preferably 48000 to52000.
 8. The method according to claim 1, wherein the content of thesoluble polymer is 10% to 50%, preferably 13% to 38%, more preferably18% to 35%, most preferably 22% to 32%, based on the total weight of themembrane.